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The update is being applied to patch a known issue that effects all Xen environments and is not AWS-specific, the company said. Amazon said that it will affect a small percentage (less than 10 percent) of the global EC2 fleet.

It is a mandatory update and must be completed by October 1. Amazon said the update is not in any way associated with what is being called “The Bash Bug” in the news today.

Not all instances will be rebooted. RightScale is reporting that it is impacting around 25 percent of the types of instances. RightScale has made a FAQ available.

AWS said that the following instance types will not be affected: T1, T2, M2, R3 and HS1. To find out if you’re being impacted, visit Amazon's “Events” page on the EC2 console. It will list pending instance reboots.

The instances that do need the update require a system restart of the underlying hardware and will be unavailable for a few minutes while the patches are being applied and the host is being rebooted.

Those instances requiring a reboot will be staggered so that no two regions or availability zones are impacted at the same time and they will restart with all saved data and all automated configuration intact.

The company issued a statement: “We understand that for a small subset of customers the reboot will be more inconvenient; we wouldn’t inconvenience our customers if it wasn’t important and time-critical to apply this update.”

The instances that need the update require a system restart of the underlying hardware and will be unavailable for a few minutes while the patches are being applied and the host is being rebooted. While most software updates are applied without a reboot, certain limited types of updates require a restart.